Object lifetime management

[kvark]

Automatic lifetime management is one of the main usability/safety gains of WebGPU API.

First problem to solve - figure out what layer is responsible for this. Since wgpu-native exposes simple IDs and it doesn't know if the user keeps any of those, it can't know when resources are destroyed, unless specifically told to do so. It can, however, track the resource usage in pending command buffers and executing on GPU. That is why I believe the lifetime should go through the following stages at wgpu-native level:

1. resource is created by the user
2. resource is used for other resources or in commands
3. user explicitly destroys the resource
4. all dependent objects and command buffers are destroyed
5. GPU has finished execution of any relevant command buffers
6. the implementation actually destroys the resource

At the same time, wgpu-rs needs to be able to avoid explicit destruction and just use RAII for that. The other clients, like Gecko, know when a resource is garbage collected and can send us appropriate message for destruction.

Second problem - how exactly to implement that tracking on wgpu-native level. I was initially thinking about have all objects to carry something like this:

pub(crate) struct LifeGuard {
    pub owned_by_user: AtomicBool,
    pub used_by_objects: AtomicUsize,
    pub used_by_commands: AtomicUsize,
    pub last_submission_index: AtomicUsize,
}

However, I later realized that we could instead enrich the Stored wrapper semantics to keep track of the usage. The sub-problem here is for Stored to not have Arc<Id> since it involves another indirection on access.